The present invention relates to a data code conversion system for converting a binary digital data code into another binary digital data code. More particularly, the invention is concerned with a data code conversion system which can profitably be used for recording data at a high density in a magnetic recording apparatus such as a magnetic disc apparatus or the like in which signal recording on a magnetic medium is performed by inverting the direction of magnetization in accordance with a signal to be recorded. Parenthetically, the phrase "magnetic recording", is intended to encompass magnetooptical recording.
As the coding (or code conversion) system suited for use in the digital magnetic recording with a high density, there has already been proposed a so-called run length limit coding system in which a limitation is imposed on the number of binary digits or bits of "0" making appearance between the adjacent binary digits or bits of "1" in the coded data resulting from the code conversion. In practical applications, there are employed a 2-7 code system in which the number of the successive binary digits of "0" (referred to as the run length) is two at minimum and seven at maximum and a 1-7 code system in which the number of the successive binary digits of "0" is one at minimum and seven at maximum.
Typical examples of these code systems are disclosed in JP-A-48-7641 (Japanese Patent Application Laid-Open No. 7641/1973) and JP-A-58-119273.
The 1-7 coding system or the 2-7 coding system of the prior art mentioned above find extensive applications in the digital magnetic recording apparatus inclusive of the magnetic disc apparatus and others. The 2-7 coding system has a code rate of 1:2 (i.e. the conversion rate corresponding to the conversion of one bit of data into two bits of a code word), wherein a code dictionary therefor consists of seven code words having lengths represented by multiples of "2" and variable in the range of 2 to 8, inclusive thereof. Of these code words, the density of the binary digits of "1" becomes maximum in such a code word in which the bit pattern of "100" is repeated (i.e. the word in which the successive run lengths are all of 2, respectively). This maximum bit density is given by 1/3. In other words, the density or frequency at which the binary digit (bit) of "1" makes appearance in the three successive bits "100 " is one. On the other hand, the density of the bits of ( "1" is minimum when the bit pattern "10000000" is repeated (i.e. when the run lengths making appearance in succession are all of 7, respectively). In this case, the density of the bit "1" is given by 1/8, meaning that the number of the bit of "1" contained in the eight successive bits of "10000000" is one. The ratio of the maximum density of the bits "1" to the minimum density thereof is obviously 8 to 3 (8:3).
The 1-7 coding system is designed for the conversion at a code rate of 2 to 3 (i.e. the rate for converting two bits of a data into three bits of a code word). In the code word obtained after this code conversion, the number of the bits "0" occurring between the adjacent bits of "1" is one at minimum and seven at maximum. There are available seven different bit patterns for the code word in this 1-7 code system.
On the other hand, in connection with reproduction of the coded data by resorting to the use of a phase synchronizing circuit in the magnetic recording apparatuses such as the magnetic disc apparatus and others, it is preferred for the reasons which will be described later on to decrease an average magnetization inverting interval rather than the maximum magnetization inverting interval while making uniform the magnetization inverting interval on an average in view of the fact that a greater phase margin can be assured.
More specifically, the run-length limited code resulting from the code conversion mentioned above is recorded in such a manner that the direction of magnetization is inverted for the binary digit value of "1" while recording of the binary digit value of "0" is performed without inverting the direction of magnetization. Upon reproduction of the coded data, a clock pulse signal is generated by a clock generating oscillation circuit in synchronism in phase with the pulse signal derived in correspondence with the inverted magnetization, wherein the individual data bits are discriminatively identified with the aid of a discriminating window prepared on the basis of the pulse interval of the clock pulse signal (which is equal to the interval between the data bits). In that case, the PLL clock generating oscillation circuit (phase-locked loop oscillation circuit) can oscillate more stably, being locked positively to the sample pulses (the pulses corresponding to the inverted magnetizations), as the average number of the pulses (the number of samples) inputted to a phase comparator incorporated in the PLL clock generating oscillation circuit is increased. Accordingly, discrimination of the reproduced data can be performed with a high reliability without involving error by virtue of the discriminating window operative on the basis of the clock pulse signal outputted from the PLL oscillation circuit mentioned above, even when the data as reproduced suffers from noise jitter. More concretely, even when the magnetization inverting interval (and hence the interval between the sample pulses) becomes transiently or temporarily longer, the clock oscillator can positively be locked in the phase to thereby oscillate in a more stable manner, as the magnetization inverting interval averaged over a plurality of data blocks becomes shorter or as variation in the average magnetization inverting interval becomes less significant. In light of the foregoing, there exists a demand for such a data coding system in which the density of the bits of "1" bringing about the inversion of magnetization upon data recording is high, the number of successive data blocks having a low density of the bits "1" (having a high density of the bits "0" to say in another way) is small and in which the density of the bits "1" averaged over a plurality of data blocks is sufficiently high without undergoing any noticeable change (i.e. at a low rate of change) even when the data block having a long magnetization inverting interval should make appearance occasionally or infrequently.
In this connection, it is noted that the 2-7 code system known heretofore suffers from a problem that the phase margin can not be increased because the density (average density) of the bits "1" changes at a relatively high rate of 8:3, as described hereinbefore.
Further, in connection with the reproduction of the coded data from a recording medium in the magnetic recording apparatus such as the magnetic disc apparatus and others, a procedure is generally adopted in which the electric signal generated upon reading the coded data is sliced at a predetermined slice level for reproducing the code words.
Now, let's consider the margin for the slice level employed for the read-out signal in the reproducing operation. As is illustrated in FIGS. 1(A) and 1(B) of the accompanying drawings, it has been found that the margin of the slice level for reproducing the signal of such patterns in which the number of the bits "0" (i.e. run length) occurring between the adjacent bits of "1" is 6-1-1-6 (00000010101000000) or 7-1-1-7 (0000000101010000000) for the four successive run lengths is extremely small when compared with other bit patterns. This can be explained by the fact that in the case of two patterns mentioned above, the peak level of the read-out signal varies significantly, as will be seen in FIGS. 1(A) and 1(B), because of remarkable variations in the successive run lengths (and hence the noticeable change in the rate of occurrence of the bit "1"). Therefore, a very important problem to be solved for enhancing the reliability of the magnetic recording apparatuses is how to reproduce such a pattern portion as presenting a small reading margin by resorting to an appropriate circuit design.
As will be appreciated from the above discussion, the 2-7 coding system or the 1-7 coding system known heretofore suffer a disadvantage that the margin for reading out the data is small due to the significant change in the density (rate of occurrence) of the bits "1" on an average or locally.